Fire Ecology Volume 10, Issue 1, 2014
doi: 10.4996/fireecology.1001014
Morgan et al.: Northern Rockies Pyrogeography
Page 14
Research Article
Northern Rockies pyrogeography:
An example of fire atlas utility
Penelope Morgan1*, Emily K. Heyerdahl2, Carol Miller3, Aaron M. Wilson1,
and Carly E. Gibson4
1
University of Idaho, Department of Forest, Rangeland, and Fire Sciences,
875 Perimeter Drive MS 1133, Moscow, Idaho 83844-1133, USA
USDA Forest Service, Rocky Mountain Research Station,
5775 US West Highway 10, Missoula, Montana 59808, USA
2
USDA Forest Service, Rocky Mountain Research Station,
Aldo Leopold Wilderness Research Institute,
790 E. Beckwith Avenue, Missoula, Montana 59801, USA
3
USDA Forest Service, Stanislaus National Forest,
19777 Greenley Road, Sonora, California 95370, USA
4
Corresponding author: Tel.: 001-208-885-7507; e-mail: pmorgan@uidaho.edu
*
ABSTRACT
RESUMEN
We demonstrated the utility of digital
fire atlases by analyzing forest fire extent across cold, dry, and mesic forests,
within and outside federally designated
wilderness areas during three different
fire management periods: 1900 to 1934,
1935 to 1973, and 1974 to 2008. We
updated an existing atlas with a
12 070 086 ha recording area in Idaho
and Montana, USA, west of the Continental Divide, 81 % of which is forested. This updated atlas was derived
from records maintained locally by 12
national forests and Glacier National
Park. Within the cold, dry, and mesic
forests that encompass 45 %, 44 %, and
11 % of the fire atlas recording area, respectively, we analyzed 3228 fire polygons (those ≥20 ha in extent and ≥75 %
forested). We discovered that both fire
extent and the number of fire polygons
were greater in the north during the
Demostramos la utilidad de los atlas digitales
de incendios analizando la cobertura de incendios forestales en bosques fríos, secos y mésicos, dentro y fuera de áreas silvestres designadas por el gobierno federal durante tres diferentes periodos de tiempo de manejo del fuego:
1900 a 1934, 1935 a 1973 y 1974 a 2008. Actualizamos un atlas existente con una superficie con registro de incendios de 12 070 086 ha
en Idaho y Montana, EUA, al oeste de la divisoria de aguas continental; 81 % de esta superficie tiene cobertura forestal. Este atlas actualizado se derivó de registros locales generados
por 12 bosques nacionales y el Parque Nacional Glacier. Dentro de los bosques fríos, secos
y mésicos que abarcan, respectivamente, el
45 %, 44 % y 11 % del área de registro del atlas, analizamos 3228 polígonos de incendios
(aquellos con ≥20 ha de extensión y cobertura
forestal ≥75 %). Encontramos que la extensión de los incendios y el número de polígonos
de incendios fueron mayores en el norte du-
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Fire Ecology Volume 10, Issue 1, 2014
doi: 10.4996/fireecology.1001014
early period and greater in the south
during the late period, but in all cases
burned in proportion to land area.
Over the 9 731 691 ha forested fire-atlas recording area, 36 % of 10 000 randomly located points burned at least
once, 7 % burned twice, and fewer
than 1 % burned three or more times.
Of these same points, disproportionately more burned inside wilderness
than outside. These points burned in
proportion to land area by forest type
and generally by slope, aspect, and elevation. Analysis revealed that despite extensive fires early and late in
the twentieth century, area burned was
likely still low relative to prior centuries, especially at low elevations and
outside large wilderness areas. The
fire atlas includes few fires <40 ha,
and its perimeter accuracy is uncertain
and likely historically inconsistent;
even so, the perimeters are georeferenced and, because they include the
entire twentieth century, can serve to
bridge past and future fire regimes.
Fire atlases are necessarily imperfect,
but they remain useful for exploring
the pyrogeography of modern fire regimes, including how the spatial distribution of fire varied through time
with respect to landscape controls, fire
management, and climate.
rante el primer periodo, mientras que en el sur
fueron mayores durante el último periodo, pero
en todos los casos afectaron a una superficie
proporcional al área total. La superficie total
con cubierta forestal y registro de incendios fue
de 9 731 691 ha; de 10 000 puntos distribuidos
al azar, el 36 % se quemó al menos una vez, el
7 % se quemó dos veces y menos del 1 % se
quemó tres veces o más. De estos mismos
puntos, un número desproporcionadamente
mayor se quemó dentro de las áreas silvestres
que fuera. Estos puntos se quemaron en proporción a su superficie por tipo de bosque y
generalmente por pendiente, exposición y altitud. Los análisis mostraron que a pesar de los
grandes incendios registrados a principios y finales del siglo XX, el área quemada sigue siendo baja en relación a siglos anteriores, especialmente en elevaciones bajas y fuera de las
áreas silvestres. El atlas de fuego incluye algunos incendios <40 ha, aunque la precisión de
los perímetros es incierta y probablemente
históricamente inconsistente; aun así los
perímetros están geo-referenciados y como incluyen todo el siglo XX, pueden servir para
enlazar los regímenes de fuego pasados y futuros. Los atlas de fuego son imperfectos, pero
pero siguen siendo útiles para explorar la pirogeografía de los regímenes de fuego modernos,
así como entender cómo varía la distribución
espacial del fuego a través del tiempo con respecto a los controles del paisaje, el manejo del
fuego y el clima.
Keywords: fire atlas, fire history, fire regimes, northern Rocky Mountains, wildfires
Citation: Morgan, P., E.K. Heyerdahl, C. Miller, A.M. Wilson, and C.E. Gibson. 2014. Northern
Rockies pyrogeography: an example of fire atlas utility. Fire Ecology 10(1): 14–30. doi:
10.4996/fireecology.1001014
INTRODUCTION
Fire atlases, also called digital polygon fire
histories, are compilations of annual fire perimeter records. On many federal lands in the
western United States, historical fire perimeters have been digitized from hand-drawn
maps and from fire incident reports. More recently, they have been inferred from satellite
imagery (e.g., the Monitoring Trends in Burn
Severity project; www.mtbs.gov), infrared
sensing, or hand-held global positioning system (GPS) receivers, and integrated into a geographic information system (GIS). These digi-
Fire Ecology Volume 10, Issue 1, 2014
doi: 10.4996/fireecology.1001014
tized records contain georeferenced fire perimeters; thus, they differ from point records of
fire occurrence and size (e.g., those used by
Westerling et al. 2006), as well as from records
of annual area burned compiled for national
forests or regions (e.g., those used by Littell et
al. 2009). When combined with other georeferenced records such as topography and vegetation type, georeferenced fire perimeters can
be used to describe fire patterns over all parts
of the landscape through time, thus better informing our understanding of what determines
those fire patterns (Rollins et al. 2001, 2002).
Both ignition and area burned are influenced
by complex, multiscale interactions among fuels, climate, weather, topography, and human
action (Krawchuk et al. 2009, Parisien and
Moritz 2009, Parisien et al. 2011). Characterizing this variation, termed pyrogeography, is
critical to understanding both the causes and
consequences of fire, and how these vary in response to climate, topography, land use, and
vegetation—all of which interact at multiple
scales. Parisien and Moritz (2009) lamented
the paucity of analyses of the “habitat” of fire
across environmental gradients, while highlighting its value in understanding and predicting the response of fire to changing climate
and fire management.
Topography is a strong driver of fire behavior and likely affects the spatial distribution
of area burned. If fires burn randomly, then
the distribution of area burned by landscape
characteristics (e.g., its topography or forest
type) should be similar to the distribution of
land area by those same characteristics. If,
however, some locations are predisposed to
burning, the patterns of distribution can highlight potential mechanisms driving past fire regimes. Many possibilities exist. For example,
the spatial distribution of fire across parts of
the western US was found to be controlled by
different factors (Barrett et al. 1991; McKelvey and Busse 1996; Rollins et al. 2001,
2002; Falk et al. 2007; Kellogg et al. 2008)
and at regional to local scales (Parisien and
Morgan et al.: Northern Rockies Pyrogeography
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Moritz 2009). In Spain, fires burned preferentially in some landscape positions (Moreno et
al. 2011), whereas in the western US, spatial
variation in vegetation, weather, and ignitions
affected burn probability (Moritz et al. 2005,
Parisien and Moritz 2009, Parisien et al. 2011).
Rollins et al. (2002) found that elevation and
aspect influenced twentieth century fire frequency in large wilderness areas, but Podur
and Martell (2009) and Bessie and Johnson
(1995) found that fire extent was little influenced by landscape characteristics. Patterns of
fire likelihood in the western US have also
been found to be significantly influenced by
people (Parisien et al. 2012).
The US northern Rockies are unusual in
having a forest fire atlas that includes the entire twentieth century (Morgan et al. 2008).
Regional and historical variation in both fire
management and climate resulted in a changing pyrogeography—fires were more extensive, for example, to the north early in the
twentieth century and more extensive to the
south late in the century (Morgan et al. 2008).
These patterns may reflect the distribution of
landscape characteristics throughout the region, differences in fire management, spatial
variations in climate (such as distribution of
lightning storms), or a combination of these
factors.
Fire management in the US northern Rockies can be separated into three consecutive
time periods (Rollins et al. 2001, Hessburg and
Agee 2003). Early in the century (1900 to
1934), fire suppression was limited and poorly
organized until fires in 1910 and in the decades
following galvanized funding and training for
fire suppression (Hessburg and Agee 2003). In
the mid twentieth century (1935 to 1973), fire
suppression became increasingly effective, fueled by military technology and the 10 AM
policy implemented in 1935, which emphasized aggressive fire suppression everywhere
(Pyne et al. 1996). Aerial fire detection and
fire suppression by smokejumpers were initiated in 1950 at bases in the region, and local
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crews were numerous and well equipped (Pyne
et al. 1996). Late in the century (1974 to present), as part of intentional land management,
some fires were not aggressively suppressed,
particularly in federally designated wilderness
and adjacent remote areas (Pyne et al. 1996).
Fire management in the region varies locally, by land management designation of wilderness versus nonwilderness and by forest
type. Wilderness areas now account for 17 %
of the area covered by the northern Rockies
fire atlas. The greater part of these wilderness
areas were federally designated in the second
half of the twentieth century—either in 1964,
with the passage of the Wilderness Act (Bob
Marshall Wilderness and Selway-Bitterroot
Wilderness: 408 646 ha and 542 748 ha, respectively) or in 1980 (Frank Church-River of
No Return Wilderness: 958 230 ha). Managers
of many, but not all, of the wilderness areas in
the region have the option to allow lightningcaused fires to burn under prescribed conditions for ecological benefit, an option employed to varying degrees. Even before designation as wilderness, these areas, being remote,
experienced less effective initial attack and
other fire suppression efforts than did areas
with roads, especially early in the century before aerial fire detection and delivery of
smokejumpers to remote fires became more
common (Pyne et al. 1996). Further, these areas have had a lower priority for fire suppression resources, especially in years of many simultaneous large fires. Because of differences
in physical environments and fire suppression
efforts, fire size distributions inside large wilderness areas differ from those outside them
(Haire et al. 2013). Additionally, outside wilderness boundaries, fire management has been
less intensive in cold forests than in either dry
or mesic forests (Agee 1993, Hessburg and
Agee 2003, Baker 2009). In dry and mesic
forests, road building, domestic livestock grazing, and timber harvesting were more intense
and began earlier than in the higher elevation,
more remote cold forests (Hessburg and Agee
2003). As a consequence, fires had become
Morgan et al.: Northern Rockies Pyrogeography
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rare in dry forests in this region by the end of
the 1800s, before fire record keeping began in
the northern Rockies (Hessburg and Agee
2003).
Climate also varied over time in this region, which may have affected pyrogeography.
Fires in each of the forest types in the northern
Rockies fire atlas are influenced differently by
climate, with fires in cold forests more likely
to be more climate-limited than fuel-limited
(Schoennagel et al. 2004). Cold forests dominate both the higher elevations where fires
burn less frequently (Schoennagel et al. 2004),
and wilderness areas where fire suppression is
less aggressive. Litschert et al. (2012) found
that in the southern Rockies, on average, nine
times more fires burned from 1991 to 2006
than from 1930 to 1950. In the northern Rockies, fires were more extensive in later decades
of the twentieth century than in the middle of
the century; the century’s first decades had
numbers similar to this later period (Morgan et
al. 2008). Westerling et al. (2006) attributed
the western US’s longer late-century (1987 to
2003) fire seasons (with many large fires), relative to those of mid-century (1970 to 1986),
to earlier snowmelt and warmer summers. Especially sensitive were forests at middle elevations in the northern Rockies. The middle period of the twentieth century was relatively
cool and had fewer extremely dry fire seasons
(Morgan et al. 2008). Analyzing fire records
from the full century can help us understand
whether recent trends are unusual, and how
they might vary geographically.
The method of recording fire perimeters
also changed during the twentieth century and
could have influenced records of pyrogeography. Early fire atlas records were derived from
hand-drawn maps of fire perimeters and fire
incident reports, whereas recent perimeters are
derived from field mapping with GPS receivers, remote sensing, or, during some large fires,
infrared mapping.
Our objective was to describe twentieth
century pyrogeography—the spatial distribution of fire by wilderness status and topogra-
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phy—in forests of the northern Rockies and to
determine whether that pyrogeography was
different in the early (1900 to 1934) versus the
late (1974 to 2008) fire management periods.
We also highlight the utility of fire atlas records for understanding where fires occur and
why. Finally, we make recommendations to
address some of the inherent limitations of
those records.
116º 30'
114º 30'
112º 30'
110º 30'
47º 30'
METHODS
45º 30'
Study Area
The fire atlas recording area is in Idaho and
Montana west of the Continental Divide (Figure 1). Most of the atlas lies within climate
divisions 4 in Idaho and 1 in Montana (NOAA
2012). The climate is continental with cold
winters (−6 °C mean January temperature 1900
to 2008; climate division average) and warm
summers (17 °C mean July temperature 1900
to 2008). Mean annual precipitation is 58 cm
(1900 to 2003), much of which falls as snow
in winter (62 % from 1963 to 1996, Serreze et
al. 1999).
The Fire Atlas
We updated an existing atlas of the dates
and locations of fire perimeters, which had
been assembled from digital polygon fire histories maintained locally by 12 national forests
and one national park for the period 1900
through 2003 (Gibson 2006, updated through
2003 by Gibson and Morgan 2009, then updated through 2008 by Gibson et al. 2014). The
atlas does not document unburned patches
within fire polygons or indicate if a single
polygon results from more than one ignition.
For our analysis, we excluded polygons less
than 20 ha in extent because small polygons
were not consistently recorded and some small
polygons were artifacts of combining records
from different land management units. This
excluded 1129 fire polygons with a total extent
of 8790 ha. We further limited our analysis to
43º 30'
Continental Divide
Montana
Wilderness
Idaho
Recording area
Cold forest
Dry forest
Mesic forest
Figure 1. The study area includes cold, mesic, and
dry forests in Idaho and Montana west of the Continental Divide. We analyzed forested area within
a fire atlas recording area derived from records
maintained by Glacier National Park and 12 national forests from 1900 to 2008, within and outside federally designated wilderness areas.
the 9 731 691 ha that were forested, equivalent
to 81 % of the 12 070 086 ha recording area.
We defined forested polygons as those with
≥75 % of pixels classified as forested (generally >10 % forest cover; LANDFIRE 2012).
This excluded 603 fire polygons in non-forest
vegetation types with a total extent of 835 805
ha. We conducted our analyses on the remaining 3228 fire polygons with a total extent of
5 326 687 ha.
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To infer the possible influence of changing
methods of recording fire over the twentieth
century, we visually examined the perimeterto-area ratio of fire polygons over time, dividing the ratio of each polygon by the area of a
circle of the same area to remove the bias of
patch size. If satellite imagery had captured
more of the complexity in fire boundaries late
in the century than hand mapping of fires did
early in the century, we would expect the perimeter-to-area ratio to increase correspondingly. In addition, if detection and mapping of
small fires improved later in the century, this
index would presumably also increase over
time.
Pyrogeography
To characterize pyrogeography, we compared (1) polygons by forest type, wilderness
status, elevation, aspect, and slope; and (2) the
distribution of a set of randomly selected
points to the distribution of subset of these
points that had burned. All spatial data had a
30 m resolution.
We assigned forest types using environmental site potential (ESP; Rollins 2009; public communication, http://www.landfire.gov/
NationalProductDescriptions19.php). The
ESP is a potential vegetation classification indicating the vegetation an area could support
based on biophysical site characteristics such
as climate, topography, and substrate. Although they are named for forest types, ESPs
reflect the biophysical template (e.g., a mesic
forest is found where both climate and site
conditions are moist, although many different
tree species could dominate depending on disturbance history). We grouped ESPs into three
broad categories (Morgan et al. 2008): cold
forests, mesic forests, and dry forests. All
three forest types occur across a broad range
of aspects. Cold forests primarily occur above
1500 m, typically on shallow, poorly developed soils and are often dominated by lodgepole pine (Pinus contorta Douglas ex Loudon),
Morgan et al.: Northern Rockies Pyrogeography
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subalpine fir (Abies lasiocarpa [Hook.] Nutt.),
Engelmann spruce (Picea engelmannii Parry
ex Engelm.), and other cold-tolerant tree species. Dry forests are usually found both above
and below 1500 m on well-drained, often poorly developed soils and are dominated by ponderosa pine (Pinus ponderosa P.C. & Lawson)
and Douglas-fir (Pseudotsuga menziesii [Mirbel] Franco). Mesic forests are usually found
below 1500 m in areas with high annual (and
growing season) precipitation and well-developed soils. Mesic forests are dominated by a
mixture of conifer species, including grand fir
(Abies grandis [Douglas ex D. Don] Lindl.),
Douglas-fir, western larch (Larix occidentalis
Nutt.), western hemlock (Tsuga heterophylla
[Raf.] Sarg.), and western redcedar (Thuja plicata Donn ex D. Don). Cold, mesic, and dry
forests occupy 45 %, 11 %, and 44 % of the
9 731 691 ha forested part of the fire atlas recording area (Morgan et al. 2008). Although
portions of some fire polygons were not forested, 90 % of the fire extent recorded as
burned in the atlas occurred in these three forest types. In addition, many fire polygons included more than one forest type. In those
cases, we subdivided polygons accordingly for
analyses including forest type as a variable.
One fifth (17 %) of the fire atlas recording
area currently lies either in federally designated wilderness areas or in Glacier National Park
(USDI 2010; Figure 1). Most of the forest
within wilderness is cold forest (63 %; Figure
1); the remainder is mostly in dry forest (34 %)
with a small amount in mesic forest (3 %). In
contrast, outside wilderness areas, nearly half
the forest is dry (47 %), with the rest cold
(40 %) or mesic (13 %).
We randomly selected 10 000 points from
the forested portion of the fire atlas recording
area that were separated by at least 42 m (the
length of the hypotenuse of a 30 m pixel, in
the digital elevation and ESP data). We chose
this distance to avoid selecting more than one
point in any 30 m pixel. Of the 3738 points
that burned between 1900 and 2008, 17 %
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burned more than once (to a maximum of 5
times), and thus we included them in our analysis more than once. The distribution of points
according to topography (slope, aspect, and elevation from a 30 m digital elevation model)
and forest type varied latitudinally in our study
area; during the early period, almost all of the
burned pixels (95 %) were north of 45º 30’ N,
whereas in the late period, more than half the
burned pixels were south of this latitude (58 %;
Figure 2). Therefore, we separately analyzed
areas north and south of 45º 30’ N. Most of
the fire atlas recording area (and our randomly
selected points) lay north of this latitude (60 %
and 62 %, respectively). We compared the distributions using Pearson’s chi-squared test
(SAS, Cary, North Carolina, USA). With large
sample sizes, P values are small (Lin et al.
2013), so with our very large sample size of
10 000 points, we assumed that the distributions differed significantly at α ≤ 0.0001.
We also compared fire polygon size distributions by wilderness status and time period,
but not by topography. Individual fire polygons often span more than one topographic
facet; that is, a single fire can burn on both
Early
1900-1934
north and south slopes. Furthermore, the complex terrain of the northern Rockies results in
relatively small topographic facets. In contrast, apportioning fire polygons by wilderness
areas is less challenging because wilderness
areas in this region are relatively large. We
classified polygons as inside wilderness when
≥75 % of the polygon was within federally
designated wilderness areas.
RESULTS
The Fire Atlas
In the forested fire atlas recording area between 1900 and 2008, total forest fire extent
was 5 326 687 ha, as computed from polygons
that were ≥20 ha and ≥75 % forested, including areas that burned more than once and unburned islands within fire perimeters. In a majority of years (90 %), fewer than 75 fire polygons were mapped, even in years when total
fire extent was high (Figure 3a, b). Fire extent
during the early and late periods was much
greater than during the middle period (Figure
3a), and a greater number of polygons also
Middle
1935-1973
Late
1974-2008
Figure 2. Total fire extent (red) and its spatial distribution varied among fire management periods.
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Fire management period
Middle
Early
Fire extent
(ha x 100 000)
12
Number of
fire polygons
(a) Fire extent
10
8
6
4
2
0
400
(b) Number of fire polygons
300
200
100
0
(c) Years with data in the fire atlas
North
IP
GLA
FLA
KOO
LOL
CLE
BIT
NP
SC
PAY
SAW
BOI
CT
Perimeter-to-area
ratio relative to a circle
(m ha -1 )
Management unit
Late
South
(d) Perimeter-to-area ratio
8
6
4
2
0
1900
1925
1950
1975
2000
Year
Figure 3. The northern Rockies fire atlas, showing fire polygons ≥20 ha in extent and ≥75 % forested.
Fire polygons were recorded by 13 land management units: Idaho Panhandle (IP), Flathead (FLA), Kootenai (KOO), Lolo (LOL), Clearwater (CLE), Bitterroot (BIT), Nez Perce (NP), Salmon-Challis (SC),
Payette (PAY), Sawtooth (SAW), Boise (BOI), and Caribou-Targhee (CT) national forests; and Glacier
National Park (GLA)
burned early and late relative to middle. The
annual number of fire polygons varied from 0
in 1901 and 1965 to 354 in 1910. The years
with the most burned polygons commonly but
not always coincided with the years of greatest
fire extent.
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No management unit recorded fire during
every year of the twentieth century (Figure
3c). The gaps in fire records do not always coincide among management units, but more
than half of the management units recorded
fire polygons in more than half of the years.
Some records were nearly complete (e.g., Lolo
National Forest) while others (e.g., Bitterroot
National Forest) had long gaps.
The fire polygons had a broad range of perimeter-to-area ratios throughout the century,
but there is no obvious long-term trend in the
annual average of this metric (Figure 3d).
Pyrogeography
Points burned in proportion to the distribution of land area by forest type and aspect in
both the northern and southern parts of the
study area, and by elevation and slope in the
north but not in the south (Figure 4). In the
southern part of the study area, low and middle
elevations burned slightly more than expected
and high elevations burned less, and steeper
(30 % to 60 %) slopes burned more than expected. Fires burned disproportionately more
inside wilderness than outside wilderness areas, both in the north and in the south.
Fire polygon size distributions likewise
varied by wilderness status (Figure 5a, b). A
greater proportion of small burned polygons
(20 ha to 100 ha) were recorded outside than
inside wilderness, both in early (1900 to 1934)
and late (1974 to 2008) periods. In contrast,
proportionately more of the 101 ha to 1000 ha
polygons burned inside than outside wilderness, both early and late in the record. Of fire
polygons 1001 ha to 10 000 ha and 10 001 ha
to 100 000 ha, a higher proportion burned outside wilderness than inside late in the record in
contrast to the early period (Figure 5a, b).
Most of the recorded area burned was in large
°
°
°
°
Figure 4. Proportion of land north and south of 45º 30’ N latitude, compared to proportion burned at
10 000 randomly located points within the forested fire atlas recording area (9 731 691 ha). Statistically
significant test statistics are indicated in bold (α ≤ 0.0001).
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Morgan et al.: Northern Rockies Pyrogeography
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polygons both inside and outside wilderness
(Figure 5c, d). In the late period, the proportion of the largest polygons (>10 000 ha) was
higher outside than inside wilderness (Figure
5c, d).
DISCUSSION
Twentieth Century Pyrogeography
Figure 5. Size distribution of forested fire polygons (≥20 ha in extent and ≥75 % forested) by time
period inside (shaded) versus outside (black) federally designated wilderness early and late in the
fire atlas record.
Burn probability and polygon size distribution from fire atlases can be analyzed across
environmental gradients to inform our understanding of fire as a key disturbance process
shaping vegetation distribution (Bond and
Keeley 2005) and landscape dynamics. The
109-year record of the northern Rockies fire
atlas enables evaluation of fire extent across
complex terrain, contrasting fire management
and varying climate (Rollins et al. 2001, 2002;
Morgan et al. 2008; P. Higuera et al., University of Idaho, unpublished report) in a region
with a rich history of fire, influence on fire policy, and both recent (Westerling et al. 2006,
Morgan et al. 2008) and projected future increase in fire extent (Littell et al. 2009, Spracklen et al. 2009).
If fire were an organism in the northern
Rockies, we would declare it more a generalist
than a specialist with respect to habitat. We
expected fires to burn preferentially in some
places on the landscape (e.g., on low-elevation, south-facing slopes), but found instead
that the distribution of fire was not strongly
controlled by topography. Parisien et al.
(2011) concluded that environmental influences on fire vary regionally. Even though rugged
topography dominates in the northern Rockies,
it did not strongly influence the distribution of
fire at the scale measured (in 30 m pixels and
across broad classes of forest type and topography). It is possible that analysis at a different scale would yield different results (Parks et
al. 2011), but this also could be related to the
type of fires recorded in the atlas, most of
which were large (>40 ha) and burned under
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relatively extreme conditions during years of
widespread fire (Morgan et al. 2008). Landscape characteristics are less influential during
extreme fire weather (Turner and Romme
1994, Bessie and Johnson 1995, Mermoz et al.
2005, Haire et al. 2013), and Dillon et al.
(2011) hypothesized that fire extent is more influenced by climate than by topography, especially in years of widespread fire when the
most area burned.
We also expected fires to preferentially
burn some forest types, but they did not. Historically, fires were more frequent in dry than
cold forests (Schoennagel et al. 2004), leading
us to expect that proportionately more area
would have burned in dry forests than in cold
forests in the twentieth century. Instead, the
fire atlas reveals that modern fires burned both
cold and dry forests in proportion to their distribution by land area and in roughly equal
amounts (Figure 4). That dry forests burned as
much as cold forests in the twentieth century
likely reflects a change in dry forest fire regimes where fires have become less frequent
as a consequence of fire exclusion (Hessburg
and Agee 2003).
We stratified our analyses into north versus
south because of the obvious differences in fire
extent recorded in the fire atlas early versus
late in the century (Figure 2). This geographic
pattern may be due to both land management
and landscape differences. Following the extensive fires of 1910 that burned in the north,
fire suppression, logging, and road building
were aggressive (Hessburg and Agee 2003),
particularly where timber was harvested in the
productive mesic and dry forests. Further, wilderness areas are more extensive to the south
and it is likely that records of fires are less
complete in wilderness and other remote terrain. Alternatively, the driver of temporal and
spatial differences could be location of lightning storms or climate changes through time.
The fire atlas reflects modern patterns that
may differ considerably from historical patterns, especially for dry forests. Tree-ring reconstructions of surface fire at 14 dry forest
Morgan et al.: Northern Rockies Pyrogeography
Page 24
sites in the fire atlas recording area averaged 8
fires per century from 1650 to 1900 (range 4 to
14 fires; Heyerdahl et al. 2008). The fire atlas
record from the same locations and area sampled had a record of many fewer fires after
1900 (range 0 to 2 fires from 1900 through
2008; Heyerdahl et al. 2008). While these
tree-ring data are from sites targeted for frequent fires, the contrast with the fire atlas data
suggests that departures from historical fire regimes occurred before 1900 in dry forests and
continue to the present.
We expected fire polygon size to be greater
within than outside wilderness, but it was not
(Figure 5). Although the proportion of fires 20
ha to 1000 ha in size within and outside wilderness was similar both early and late in the
record, there were more large fires outside
compared to inside wilderness (Figure 5). Late
in the record, there were proportionately more
large fires (1001 ha to 10 000 ha and 10 001 ha
to 100 000 ha) outside wilderness despite the
typically longer response time and lower priority for fire suppression efforts within wilderness. This pattern could arise if small fires are
less likely to be detected and mapped in wilderness areas, but could also be because nonwilderness contains proportionately more dry
forests with their longer fire season and different fuels, or fire spread could be influenced by
complex topography and previous fires (Teske
et al. 2012, Parks et al. 2014; but see Haire et
al. 2013).
Finally, we expected the quality of fire records to improve in recent decades as a consequence of improved technology, but it did not
(Figure 3d). Fire management and climate
likely influenced polygon size distribution.
Fire suppression, with the help of roads and
land uses outside of wilderness areas, likely
resulted in smaller fire polygons, but under the
more extreme climate of the regional fire years,
fires may have burned irrespective of variations in vegetation, fuels, and topography. For
these and perhaps other reasons, we did not
find the differences in fire polygon size distribution we expected through time and space.
Fire Ecology Volume 10, Issue 1, 2014
doi: 10.4996/fireecology.1001014
Multivariate analysis with continuous data
may illuminate relationships that we did not
detect with our analyses. In addition, analyses
at other spatial and temporal scales might also
be revealing (Falk et al. 2007).
Atlases Are Valuable and Useful,
if Incomplete,
Records of Twentieth Century Fire
The northern Rockies fire atlas proved useful in studying the pyrogeography of the region. However, the nature of the fire atlas data
limits possible inferences. We offer several
cautionary notes on completeness, consistency,
and accuracy in using such data—cautions
likely applicable to other fire atlases, including
those of recent decades derived from satellite
imagery or other remotely sensed data.
Although the northern Rockies atlas included 109 years, it was an incomplete temporal record, containing gaps. We must expect
that other atlases contain similar gaps, and we
caution against inferring absence of fire from
absence of data. While long gaps in the record
(e.g., the 1950s in the Bitterroot National Forest) are likely due to lack of recording or loss
of records, shorter gaps, of one to several
years, could be due to lack of reporting, lack
of fire, or both. Although we suspected that
shorter gaps probably reflect a relative lack of
fire in those years, we did not assume it; consequently, we did not attempt to weight the
data from the different management units.
Fire atlases also are likely incomplete because of a bias toward mapping large fires that
are considered more economically and socially
significant (McKelvey and Busse 1996). We
recognize that small fires in the northern Rockies were less likely to be mapped than large
ones, and that this bias would be even more
pronounced if small fires were to go undetected in the remote wilderness areas making up a
large portion of this region. This bias in the
fire atlas data introduces uncertainty into any
inferences that we can make about fire size
Morgan et al.: Northern Rockies Pyrogeography
Page 25
distributions. Inferences about size distributions are further limited by inconsistencies in
the minimum mapping unit (i.e., the size of the
smallest fire polygons mapped), which likely
varied through time and by management unit
(McKelvey and Busse 1996, Minnich and
Chou 1997, Morgan et al. 2001, Rollins et al.
2001, Gibson 2006).
Inconsistent methods of mapping fire perimeters in the northern Rockies fire atlas are
another source of uncertainty. Prior to the mid
1930s, fires in the atlas were primarily mapped
from stand ages interpreted from aerial photographs (J.H. Logsdon, USDA Forest Service
Idaho Panhandle National Forests, personal
communication; Rollins et al. 2001), but fire
reports and maps, newspaper accounts, journals, and interviews were also used (Gibson
2006). Mapping has improved through the
century and perimeters are now often interpreted from satellite imagery. However, even
today, mapping methods vary widely and range
from satellite-derived data and airborne imagery (including infrared mapping of perimeters
of actively burning fires) to hand-drawn perimeters. Depending on the method and the person employing it, simple or complex perimeters could result. We did not find a systematic
bias in the northern Rockies atlas in shape
complexity of the fire polygons through time.
Other atlases may differ in this respect; this
could be revealed with simple analyses of the
complexity in mapped fire perimeters.
Perimeters in fire atlas data simplify reality. Unfortunately, unburned islands within fire
perimeters are not mapped (Kolden and Weisberg 2007), and atlases like the one from the
northern Rockies contain fire perimeters, but
not burn severity. Because atlas perimeters do
not discern unburned islands, they likely overestimate the area burned by any single fire, although the degree to which this is true is generally unknown (Kolden et al. 2012). The perimeters are approximate and ground truthing
is necessary before relying on them for many
applications. Accuracy of most perimeters is
Fire Ecology Volume 10, Issue 1, 2014
doi: 10.4996/fireecology.1001014
unknown and likely varies through time and
among management units. For example, Shapiro-Miller et al. (2007) found that the fire perimeters in the northern Rockies fire atlas
agreed with fire scar records, and Farris et al.
(2010) similarly found that fire atlas and fire
scar records were in strong agreement. Yet
Kolden and Weisberg (2007) found poor agreement between manually mapped fire perimeters (created through a number of different undocumented methods) and Landsat-derived
perimeters, especially in areas of complex topography. Fire atlas data should be regarded
with healthy skepticism and supplemented
with other sources of information whenever
possible.
Despite these weaknesses, atlas data can be
very useful. These “wall-to-wall” georeferenced landscape fire histories complement remote sensing, and tree-ring and other fire proxies. While fire atlases typically do not include
all small fires, they do capture the larger fires
that account for the majority of fire extent (Shapiro-Miller et al. 2007, Farris et al. 2010) and
provide one of the only sources of information
about temporal change in fire polygon size distribution prior to the advent of remote sensing
from satellite imagery and aerial photography.
They are georeferenced, and so can be linked
to other georeferenced data such as topography,
vegetation type, climate, and weather. For instance, they are useful in evaluating where and
why fire regimes change across time and space
(Morgan et al. 2001, Rollins et al. 2002). They
are also useful for elucidating the causes of
temporal and spatial variation in twentieth century fire regimes (e.g., Morgan et al. 2008).
They can also be used to identify sites for
studying forest dynamics for conservation and
restoration. In many regions, digital polygon
fire histories are the only record of the location
and extent of fires throughout the twentieth
century, a time period that encompasses different fire management practices, multiple forest
Morgan et al.: Northern Rockies Pyrogeography
Page 26
vegetation types, and multiple land uses, all of
which influence fire regimes.
Because they complement fire records from
prior centuries such as charcoal and tree rings,
fire atlas data form a bridge between the past
and the future. Simulation models of landscape
dynamics can be informed by, parameterized
with, and tested against fire atlas data (Miller
2007). Models predicting burn probability also
rely upon fire size distributions (e.g., Parks et
al. 2011) and, despite the biases discussed
above, fire atlas data can provide site-specific
inputs to many of these models.
As a consequence, maintaining and improving fire atlas records is valuable and that
value will only continue to increase. The National Wildfire Coordinating Group (2006) adopted national interagency standards for digital
polygon fire histories, but did not include all
of the fire descriptors we think are ecologically
or methodologically significant, such as source
of perimeter location (e.g., GPS, satellite,
hand-drawn polygon), fire cause (lightning or
human), fire start and end date, fire management actions, minimum mapping unit, assumptions made while gathering and developing the
data, data dictionaries (i.e., definition of values
in an attribute column), software used to digitize fire perimeters, and process steps used to
assemble polygon fire histories. Ideally, they
would also capture unburned islands within
fire perimeters and daily fire progression.
Fire atlas data, although imperfect, offer
opportunities for analyzing fire patterns, relative to spatially rich climate and weather data,
to enrich our understanding of fire as a spatial
process varying through time. Understanding
the pyrogeography of fire across large, complex landscapes and through time periods of
changing fire management and climate will inform the mapping of fire likelihood under future scenarios, and provide useful inputs and
tests of simulation models.
Morgan et al.: Northern Rockies Pyrogeography
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Fire Ecology Volume 10, Issue 1, 2014
doi: 10.4996/fireecology.1001014
ACKNOWLEDGEMENTS
This project would not have been possible without the help of those within the US Forest Service and National Park Service who maintained and shared fire atlas data. This research was supported in part by funds provided by the Rocky Mountain Research Station, Forest Service, USDA,
under Research Joint Venture Agreement 04-JV-11222048021, the USDA/USDI Joint Fire Science Program as Project 03-1-1-07, and the University of Idaho. B. Davis, L. Shapiro-Miller, M.
Rollins, A. Pocewicz, and L. Lentile shared valuable insights with us throughout the project. C.
Stevens-Rumann assisted with statistical analysis. We appreciate constructive reviews from P.
Higuera and two anonymous reviewers.
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